Mobile communication systems adopting a digital modulation method have been actively developed in recent years. Use of a more efficient amplifier in a transmission system in order to save power at radio terminals tends to invite more non-linear distortions, which should be thus compensated.
One of the methods of compensating the non-linear distortions uses a value of transmission base-band signal for referring to a distortion compensation table, thereby compensating non-linear distortions both in amplitude and phase. In this method, feedback of parts of transmission signals and update of the distortion compensation table allow the compensation to follow the changes in characteristics of the amplifier.
The foregoing instanced transmission device orthogonally demodulates a feedback signal, and a resultant signal undergoes an analog-digital (A/D) conversion to form a digital feedback signal, which is then compared with orthogonal-modulation signal to be transmitted, thereby updating the distortion compensation table. This transmission device is disclosed in Japanese Patent Non-Examined Application Publication No. H06-37831.
A conventional transmission device as discussed above feeds back a signal including an output modulation signal to be supplied outside and a non-linear distortion component created in transmission system circuits, so that an A/D converter of a feedback system circuit needs to have wide dynamic range performance.